I recently came across TarDisk, an interesting combination of hardware and software wizardry which uses the often unused SDXC slot on a Mac laptop to increase the total storage space. Considering that I am constantly running out of drive space on my work computer, this struck me as a very useful thing to assuage the perpetual freespace problem, but I was even more intrigued by the associated Pear software which combines the laptop's and the TarDisk's space into one logical unit, much like the Fusion drive in modern iMacs which take advantage of the speed of an SSD and the cheapness and spaciousness of HHD. I have not found any specifics on how Pear manages to merge the SD card with the computer's SSD, but I would not be surprised if it is making use of OS X's Core Storage technology to combine multiple disks into a single logical volume.
This got me wondering if it would be possible to do something similar by combining two connected USB disks into a single drive by using Core Storage. Indeed, this is possible. To do so involves running two commands from the command line using the diskutil utility.
diskutil coreStorage create LOGICAL_VOL_GROUP_NAME DRIVE_1 DRIVE_2 diskutil coreStorage createVolume lvgUUID type name size
LOGICAL_VOL_GROUP_NAME is the name you wish to assign to the new volume. DRIVE_1 and DRIVE_2 are the paths to the drives. Use the command diskutil list to get the path name of the drives (e.g. /dev/disk2). lvgUUID is returned after creating the new group. The output should be similar to the following:
diskutil coreStorage create FusedUSB /dev/disk4 /dev/disk5 Started CoreStorage operation Unmounting disk4 Repartitioning disk4 Unmounting disk Creating the partition map Rediscovering disk4 Adding disk4s1 to Logical Volume Group Unmounting disk5 Repartitioning disk5 Unmounting disk Creating the partition map Rediscovering disk5 Adding disk5s1 to Logical Volume Group Creating Core Storage Logical Volume Group Switching disk4s1 to Core Storage Switching disk5s1 to Core Storage Waiting for Logical Volume Group to appear Discovered new Logical Volume Group "BF21D233-99CC-8AD9-853D-4339E765UI09" Core Storage LVG UUID: BF21D233-99CC-8AD9-853D-4339E765UI09
Example
diskutil coreStorage create FusedUSB /dev/disk4 /dev/disk5 diskutil coreStorage createVolume BF21D233-99CC-8AD9-853D-4339E765UI09 jhfs+ FusedUSB 100%
To revert these changes, run the command
diskutil coreStorage delete lvgUUID. Keep in mind that creating and deleting Core Storage volumes is a destructive process and will delete any content that is already existing on the drives.